Chemical vapor deposition (CVD) of a layer of material onto a substrate is a well-known, but often complex, art. One example of a substrate is a single crystal silicon slice, or wafer, used in the manufacture of semiconductor devices. Such wafers are typically 100-125 mm diameter and are expected to be produced in excess of 200 mm diameter in the future. These silicon wafers are approximately 0.5 mm thick. Heating such substrates rapidly to high chemical vapor deposition process temperatures (900-1300.degree. C.) and cooling them to room temperature creates major technical problems for the semiconductor industry.
Material deposited on a single crystal wafer may be epitaxial (having the same crystal orientation as the wafer), polycrystalline (having many regions of different crystallographic orientations), or amorphous (having essentially no crystalline structure).
The invention described here applies to reactors specifically designed to deposit epitaxial silicon films on a single crystal silicon wafer; however, the invention also may be used for reactors to heat any thin, flat substrate to a high temperature for the purpose of depositing a single crystal, polycrystalline or amorphous film.
In the present art, wafers are placed on a carrier or susceptor which is heated to 900-1300.degree. C. in a reactor. Process gases are continuously introduced into the reactor process enclosure or chamber to react on the heated susceptor and wafers for the deposition of material upon the wafers. The gaseous by-products are exhausted from the chamber. Process gases are then purged from the chamber, and the susceptor with wafers is cooled in order to remove the wafers.
A problem with CVD reactors is the undersirable crystal defects which can occur in both the silicon wafer and the deposited epitaxial silicon layer. This is true especially for the larger substrates, 100 mm in diameter and larger. These defects are caused by induced thermal stress caused by temperature gradients in the wafer. The gradients are, in turn, caused by nonuniform heating of the wafers.
To solve this problem of nonuniform heating, various types of heating methods have been used. Three methods of heating have been used individually or in combinations to heat the susceptor in the reactor: (1) induction heating with the coils inside and/or outside the process chamber; (2) resistance heating with the heater elements inside and/or outside the chamber; and (3) radiant heating by infrared lamps placed outside the chamber. Sometimes these methods were combined with reflective shrouds placed on the process chamber walls or on the outside of the process chamber.
These heating methods have met with varying degrees of success but reduction, and possibly complete elimination, of slip remains a constant goal.
Another problem faced by all reactors is the maintenance of the process chamber walls at substantially lower temperatures than the heated susceptor to minimize deposits on the chamber walls. These deposits on the process chamber walls are a problem because they can flake off and contaminate the cleanliness of the substrates, which is vital in semiconductor manufacturing.
To cool the process chamber walls, present CVD reactor designs require elaborate cooling systems for the walls. Typically air is forced against the walls in a constant flow by a sequence of baffles and air pump(s).
The present invention avoids or substantially mitigates the problems above. Substrates are heated with minimum amounts of temperature gradients to avoid stress. Crystal slip and dislocations in the deposited layer and substrates are thus avoided. Additionally, the present invention provides for a solution to the problem of reactor wall cooling.